Lithographic printing plate and process of preparing the same



April 7, 1925.

J. KRAKES LITHOGRAPHIC PRINTING PLATE AND PROCESS OF PREPARING THE SAMEFiled m6. '1. i922 L10 I I I .4-J 16 1:: n :1 1f E1 m s, w z

A f. H P. )(I Y f I 10 V a o- Patented Apr. 7, 1925.

UNITED STATES PATENT OFFICE.

JAMES KRAKES, or CHICAGO, .ILLINoIs.

LITHOGR-VAPHIC PRINTING PLATE AND PROCESS or PREPARING THE SAME.

I Application,filedDeeember 1, 1922. Serial No. 604,214.

To alltuhom itgnay concern; H

Be it known that I, JAMEs"KRAK Es, a citizen 'of the.;Bep'ublic ofCzechoslovakia, and a resident ofthe city of Chicago, in the county ofCook and State of Illinois, have invented certain new and usefulImprove- Inentsin a Lithographic Printing Plate and Process of Preparingthe Same; and I do hereby declare that the following is a full,

clear, and exact description of the'same, reference being had to theaccompanying drawings, and to the'numerals of reference marked thereon,which form a part of'this specification" j I Heretofore whenlithographic printing plates have been made by means of a coveringofasphalt'on stone or metal, .the retouched portions of the plates haveproduced' an undesirable grayish color in the prints which is due'to anirregularity in the outline of the dots. i

It is an object of the present invention 'to k prepareYajpliotolithographic plate "which n will. avoidflthei,undesirab'lelfdi's'e esu i i I 1, .thedrawk ss may be retouched by the usual. washi'nche'mical and yet "will'leave the dots'in the retouched portions of auniformi'shape and with sharp cleancut edges.

It is a furtherobj'ectfof thisinvention to Prepar p dt e' r p i plat-in, which the as halt is" separated. from the stone or metal yajl'ayerfof material whichwill prevent. the. asphalt "from soaking intothe stone or spreading along the surface of thefnletal.

, is a further objector this inventionto provide ainetliod for reducingthe] portions in. 'a ,photolithtjgraphic ,plate 1 print too darktreatment lQf .the'jpl'atej that ray-is h color thatjhas heretofore beepro uced by-such retouched portion 3' f i Qther, and; f

fires o pas n 1 w n The. invenfi ili (ma i r rtedi rmi i more fullydescribed.

fi e neg tivethe present invention.

As shown on the drawings:

The printing plate consists of a base of lithographic stone, zinc orother metal ordinarily used for printing plates. In the preparation of alithographic plate according to my invention, a layer 11 of protectivematerial is first spread upon the stone. Preferably thls is a materialthat is. at first rather soft but hardens upon exposure to light. Any ofthe usual gums or gum-like g substances that harden upon exposure ordrying may be used. For example, a preparation of gelatin, albumen-water(White of egg), and ammonia, together with potassium dichromate, may bespread upon the stone and then exposed to light either before or duringthe drying. This exposure (ill to light may be made during thepreparation of the plate as just indicated, or the exposure through thenegative when copying the photograph may be made to serve.

If the exposure is made during the preparation of the plate and beforethe use of a negative, the better practice is to give the stone a secondcoat of the protective material in order to be sure that an abundance ofmaterial capable of being acted upon by the light is present.

When the layer 11 of protective material has hardened, the usual layerof asphalt, with which workers in this art are familiar, is spread onthe layer 11 as shown at 12. Because of the presence of the protectivelayer 11, this asphalt does not soak into the stone but remains whollyabove the layer 11. A negative 13 is then placed above the stone,

as shown in Figure 1 and the stone with solution of turpentine. Wherethe asphalt has been exposed to light, it does not readily dissolve inthe turpentine, but under the opaque parts of the negative the asphaltremains unaffected by light and will be dissolved in the turpentine. Theresult is the sort of a plate illustrated in Figure 2 where the stone ormetal 10 forming the base is covered by the protective layer 11 on whichthe dots corresponding to the transparent parts of. the negative arerepresented by specks or islands or patches of asphalt, as isillustrated at 14 in Figure 2.

In the most frequent application of my invention the negative will be ofthe type made by means of a screen, so that the specks or patches ofasphalt 14 will be distributed in a more or less regular pattern overthe plate. These cakes will be larger where the final picture is to bedarker and will be smaller where the picture printed from the plate isto be lighter. The usual photographic practices do not give a picturewhich is satisfactory in all parts, and the operator must retouch bychanging the size of the dots in portions of the plate in order toproduce an artistically satisfactory result. This is accomplished bytreating those portions of the late which print too dark with somechemical that will dissolve away a portion of the asphalt. Theusual wayof doing this is by applying the chemical locally with a wad of cottonor other convenient vehicle.

The plate before retouching is represented by Fig. 6, wherein blackportions 14 standfor the patches or dots of asphalt. Retouching, asheretofore performed, has resulted in an irregular or ragged outline ofthe dots or patches. When the asphalt is mounted on stone, thisirregularity is largely caused by the action of the asphalt in soakinginto the grains of the stone, so that the chemical acting upon theasphalt is not the only agency diminishing the size of the patches, butthere is a tendency to undercut the edges of each patch of asphalt bythe combined action of the dissolving chemical and the absorbing orcapillary action of the stone. When the asphalt is applied on metal,such as zinc, there is a tendency for the semi-dissolved asphalt tospread in a film over the metal, and this tendency has the same eifectupon the edges of the patches and results in the same irregular shapeddots.

When the protective layer 11 is present, there is nothing at the base ofthe edge walls of the several patches 16 of asphalt illustrated inFigure 3 to cause the asphalt to be removed any faster there. The actionof the chemical removes asphalt equally along the Whole height of theside surface of the patch. Consequently the edges remain vertical wallsand the resulting patches are of the shape shown at 16 in Fig. 7.

"Therefore the edges of the dots which will be formed when the print ismade will be sharp and smooth and not either irregular or fringed. Theabove-mentioned washing, retouching, local reduction or localdevelopment (these terms are used interchange ably here) is accomplishedby any of the chemicals known to Workers in this art. I have employed toadvantage a mixture of turpentine and benzene. Whatever chemical beemployed for this purpose, the new character of the edges of the patchesof asphalt and so the new character of the edges of the dots in thefinal print results from the protective layer 11.

After the retouching is completed, I treat the Whole plate with asolution of gum arabic and nitric acid for etching. This solution actsonly where the protectivelayer 11 is not covered by asphalt, so thatthere results the ,plate illustrated in-Figure 4, in which the stone 10is exposed except for the parts covered by the patches. At

'each of these dots a small piece 17 of the protective material remainscovered by a patch 16 of asphalt. If this etching is carried a littlefurther, the stone or metal 10 may be slightly etched between the dots,but this is not necessary for the carrying out of my invention.

After the etching all of the asphalt is removed by a further treatmentwith any of the usual solvents for asphalt-benzine, turpentine, benzene,or a mixture thereof. The result is the plate shown in Figure 5, inwhich the patches 17 of protective material stand up above the generalsurface of the stone or metal 10. When this plate is used for printing,the projecting patches 17 receive the ink and give the impression, whilethe stone itself acts solely as a support for the dot-forming patches.

I am aware that numerous details of construction may be varied through awide range without departing from the principles of this invention, andI therefore do not purpose limiting the patent granted otherwise thannecessitated by the prior art.

I claim as my invention:

1. A photolithographic plate comprising a foundation of lithographicstone, a layer of hardened gum-like material disposed upon the stone,and a layer of sensitive greasy material separated from the stone bysaid layer of hardened gum-like ma terial.

2. A photolithographic plate comprising a layer of light sensitiveasphalt, a foundation layer, and an intermediate layer of materialimpervious to asphalt.

3. A process of preparing a printing plate which comprises covering afoundation layer with a mixture of gelatin, albumen and a chemical whichwill cause these substances to harden when exposed to light, exposingsaid layer to li ht, covering said layer with a layer of lig t-sensi-'tive asp halt, exposing the asphalt to light through a negative,developing the result ing latent image in the asphalt, treating theportions of the plate which wouldprint too dark with a reducing compoundwhereby the size of the patches of asphalt is reduced, etching the plateto remove the hardened layer between the patches of as )halt, andfinally washing all asphalt from the plate.

4. The process of. locally reducing the tone of a photolithographicprinting plate which consists, in pre )aring a printing plate consistingof patches of asphalt, one for each dot in the finished picture,separated from the foundation by a la er of material in'ipervious toasphalt, an the selected portions of the plate with a n'iaterial thatwill dissolve asphalt whereby the size of the patches of asphalt will bereduced without producing lrregularities in the edges of said )atches.

In testimony whereo I have hereunto subscribed my name in the presenceof two subscribing witnesses.

JAMES KRAKES.

Witnesses: 1

Crmnuzs W. IIILLS,J1., Osoan I'IARTMANN.

washing

